1. Field of the Invention
The present invention relates to an electrical steel sheet provided with an insulating coating, specifically to such an electrical steel sheet which does not contain toxic compounds such as hexavalent chromium and can be produced by low temperature-baking, which is capable of stress relief annealing and has good solvent resistance. The invention further relates to the process of making the electrical steel sheet.
2. Description of the Related Art
Not only surface insulation but other convenience characteristics in processing/molding, storage and use are required of insulating coatings on electrical steel sheets used for motors and transformers. The required characteristics include punchability, TIG welding properties, adhesion property, corrosion resistance, solvent resistance, heat resistance, anti-blocking properties, anti-tension pat properties, and retention of corrosion resistance and sticking resistance after stress relief annealing.
Electrical steel sheets are subjected to stress relief annealing at 750 to 850xc2x0 C. in many cases in order to improve the magnetic characteristics of the sheet after stamping. Insulating coatings are accordingly often required to withstand stress relief annealing. Accordingly, various insulating coatings have been developed for specific electrical steel sheets used in particular ways.
Insulating coatings are usually divided into three kinds:
(1) an inorganic coating which withstands stress relief annealing and has good welding properties and heat resistance.
(2) a semi-organic coating which withstands stress relief annealing and intends to achieve both good punchability and good welding properties, and
(3) an organic coating which is limited to specific uses and cannot be annealed.
Among them, coatings (1) and (2) withstand stress relief annealing and are useful as general purpose products. In particular, chromate base insulating coatings containing an organic resin can be formed in one step comprising one coat and one bake, and have particularly excellent punchability as compared with that of an inorganic insulating coating. Such coating is therefore widely used.
A production process for an electrical steel sheet having a chromate base insulating coating is disclosed in, for example, Japanese Examined Patent Publication No. 60-36476. A processing liquid is applied on the surface of a base steel sheet. The processing liquid is prepared by blending a bichromate base aqueous solution containing at least two kinds of divalent metals with a resin emulsion having a vinyl acetate/VEOVA ratio of 90/10 to 40/60 as an organic resin in an amount of 5 to 120 parts by weight in terms of solid resin and an organic reducing agent in an amount of 10 to 60 parts by weight each per 100 parts by weight of CrO3 contained in the aqueous solution described above. Baking is carried out conventionally.
This electrical steel sheet, provided with an insulating coating, satisfies various performance requirements including corrosion resistance and solvent resistance. However, a chromate base coating has to be baked at a relatively high temperature in order to reduce hexavalent chromium to trivalent chromium in order to insolubilize it. Baking at high temperatures increases cost and energy consumption, and reduction in processing rate.
In the case of a semi-organic coating containing a resin, the resin degrades under baking at high temperatures, damaging the intrinsic performance of the resin. Further, hexavalent chromium causes concern about the problem of environmental pollution and involves cost expended for exhaust processing and waste solution processing.
Semi-organic insulating coatings contain a resin with phosphate added as a principal component. However, phosphate has to be baked at high temperatures after coating in order to promote dehydration of phosphate to insolubilize it. It therefore faces the same problem as the chromate base coating.
Some insulating coatings are capable of being baked at relatively low temperatures. A method is known in which latent heat of continuous annealing is utilized to form a coating before skin pass rolling to thereby form a coating for preventing sticking in stress relief annealing. Japanese Examined Patent Publication No. 59-21927 shows a method using an aqueous solution prepared by adding a water-soluble or emulsion-type resin with an inorganic colloidal material added as a principal component is applied, and then skin pass rolling is carried out. This method makes it possible to carry out baking at low temperatures with certainty as compared with a chromate base or a phosphate base coating, wherein a film-forming reaction for insolubilizing water soluble materials has to be promoted in order to prevent sticking. No such step is necessary for inorganic colloidal materials. Among other colloidal materials, silica completes the dehydration reaction at a reduced temperature and therefore is advantageous in low temperature-baking.
Japanese Unexamined Patent Publication No. 54-31598 discloses an electrical steel sheet provided with a heat resistant and sticking resistant coating containing organic material with silica gel added as a principal component. This is done by applying a processing liquid comprising silica hydrosol and an organic material and heating it at 100 to 350xc2x0 C., and surface treatment. This is an example of a semi-organic insulating coating capable of baking at relatively low temperatures and containing no chromic acid.
However, while the insulating coatings formed by the conventional methods described above are effective for preventing sticking in skin pass rolling and stress relief annealing, they have inferior solvent resistance. In processing, electrical steel sheets often contact organic solvents. This happens during rinsing with solvents, and contacts with cooling media (flon and the like) and various oils (punching oil, insulating oil and refrigerator oil). Therefore the insulating coatings of a good electrical steel sheet have to have good solvent resistance in addition to the other qualities heretofore discussed.
As is apparent from the examples in Japanese Unexamined Patent Publication No. 54-31598, no rust was produced in a wet test in a set of comparative examples containing chromate, but pitting corrosion was caused in all of the examples of the invention. Corrosion resistance is not described in Japanese Examined Patent Publication No. 59-21927, and therefore we investigated the performances of its electrical steel sheets. We have found that the corrosion resistance and solvent resistance of those sheets did not satisfy the performance parameters of chromate base general purpose coatings.
Further, the conventional methods described above result in inferior performance upon exposure to steam. Electrical steel sheets are often shipped through geographic locations having high temperature and high humidity. Further, when the electrical steel is incorporated into a motor and the motor is heated to a high temperature, in the presence of high humidity, resistance to steam is required in many cases.
As shown in conventional techniques, inorganic colloidal silica has excellent heat resistance and is very effective for preventing a steel sheet from sticking. However, silica has had the defects that silica alone has weak adhesion property to steel sheet, and has inferior lubricating properties and inferior punchability. It also has a weak covering capability and allows corrosion readily to occur. On the other hand, organic resins have characteristics opposed to those of inorganic colloidal silica. While organic resins have excellent punchability and adhesion property, they have inferior heat resistance. Accordingly, an insulating coating of an organic-inorganic mixed composition intended to have both advantages has been developed. As described above, however, many important coating characteristics needed for electrical steel sheets have not yet been attained.
One object of the present invention is to provide an electrical steel sheet provided with an insulating coating which can be produced by baking at low temperatures, and is capable of stress relief annealing, and has excellent solvent resistance, and contains substantially no objectionable chromium component.
Another object of the present invention is to provide an electrical steel sheet provided with an insulating coating which can be produced by baking at low temperatures and is capable of stress relief annealing and which has excellent corrosion resistance.
Another object of the present invention is to provide an electrical steel sheet provided with an insulating coating which can be produced by baking at low temperature and is capable of stress relief annealing and which has excellent steam exposure resistance.
Another object of the present invention is to provide a process for producing a non-oriented electrical steel sheet which can be produced by baking at low temperature and is capable of stress relief annealing, and which has excellent punchability and sticking resistance after annealing.
Further, the present invention provides an electrical steel sheet having an insulating coating which is excellent in all of the characteristics necessary for a variety of the performance criteria of electrical steel sheet, including adhesion property, sticking resistance and good film-forming and welding properties.
The present invention provides an electrical steel sheet fulfilling the foregoing objects. It is capable of stress relief annealing and has excellent solvent resistance and has an insulating coating containing a resin and an inorganic colloid which comprises silica or alumina or alumina-containing silica.
It can be made by baking the insulating coating at a low temperature, that is, a steel sheet temperature of about 50 to 250xc2x0 C. When the inorganic colloid is silica, the insulating coating contains at least one alkaline metal selected from the group consisting of Li, Na and K in an amount of about 0.1 to 5 parts by weight expressed as M2O (M: alkaline metal) per 100 parts by weight of silica expressed as SiO2.
Preferably, Cl is present in the insulating coating in an amount of about 0.005 part by weight or less, and S is present in an amount of about 0.05 part by weight or less each per 100 parts by weight of silica expressed as SiO2; and silica is present in an amount of about 3 to 300 parts by weight, expressed as SiO2, per 100 parts by weight of the resin.
It is further preferable that the resin contained in the insulating coating has a glass transition temperature of about 30 to 150xc2x0 C.
In the process of applying a coating liquid to the steel sheet, water is present as a solvent in which about 30 to 300 parts by weight of a colloidal silica solid material is blended with 100 parts by weight of a water base dispersed resin solid material, and in which the surface area (specific areaxc3x97solid matter weight) of the colloidal silica solid particles is controlled to about 0.2 to 10 times the surface area (specific areaxc3x97solid matter weight) of the solid resin particles. The coating liquid is baked on the steel sheet and an excellent coated electrical steel sheet is obtained.
The inorganic colloid contained in the insulating coating can be alumina, and the resin has a glass transition temperature of about 30 to 150xc2x0 C. The inorganic colloid contained in the insulating coating can be alumina-containing silica, and the resin also has a glass transition temperature of about 30 to 150xc2x0 C. An organic acid is preferably present in the insulating coating as a stabilizing agent; the colloid may be alumina or alumina-containing silica in an amount of about 3 to 300 parts by weight expressed as Al2O3+SiO2 per 100 parts by weight of the resin; and the amount of alumina contained in the insulating coating is about 0.01 to 500 parts by weight expressed as Al2O3 per 100 parts by weight of silica expressed as SiO2.
The amount of the insulating coating on the electrical steel sheet of the present invention is preferably about 0.05 to 4 g/m2.